Root-associated microbiota provide great fitness to hosts under environmental stress. However, the underlying microecological mechanisms controlling the interaction between heavy metal-stressed ...plants and the microbiota are poorly understood. In this study, we screened and isolated representative amplicon sequence variants (strain M4) from rhizosphere soil samples of Trifolium repens L. growing in areas with high concentrations of heavy metals. To investigate the microecological mechanisms by which T. repens adapts to heavy metal stress in abandoned mining areas, we conducted potting experiments, bacterial growth promotion experiments, biofilm formation experiments, and chemotaxis experiments. The results showed that high concentrations of heavy metals significantly altered the rhizosphere bacterial community structure of T. repens and significantly enriched Microbacterium sp. Strain M4 was demonstrated to significantly increased the biomass and root length of T. repens under heavy metal stress. Additionally, L-proline and stigmasterol could promote bacterial growth and biofilm formation and induce chemotaxis for strain M4, suggesting that they are key rhizosphere secretions of T. repens for Microbacterium sp. recruitment. Our results suggested that T. repens adapted the heavy metal stress by reshaping rhizosphere secretions to modify the rhizosphere microbiota.
Display omitted
•High concentrations of heavy metals result in significant enrichment of Microbacterium sp. in rhizosphere soil of T. repens.•Microbacterium sp. significantly increased the biomass and root length of T. repens under heavy metal stress.•L-proline and stigmasterol were key rhizosphere secretions of T. repens for recruiting Microbacterium sp.
Copper (Cu) is an essential element, however it’s excess into the environment causes detrimental effect on plant and risks for public health. Four Cu and drought tolerant ...1-aminocyclopropane-1-carboxylate (ACC) deaminase producing rhizobacteria were isolated from the roots of Trifolium repens L. growing on Cu smelter contaminated soils, characterized and identified based on 16S rRNA gene sequencing. A consortium of high ACC deaminase (53.74 μM α-ketobutyrate mg−1 protein h−1) producing bacteria Pseudomonas sp. strain TR15a + siderophore producing Bacillus aerophilus strain TR15c significantly (p < 0.05) produced better results for multiple-metal tolerance including Cu (1750 mg kg−1), antibiotic resistance (ampicillin, kanamycin, chloramphenicol, penicillin, tetracycline, and streptomycin) and plant growth promoting attributes (phosphate solubilization: 315 mg L−1, indole-3-acetic acid (IAA) production: 8 mg L−1, ammonia and hydrogen cyanide production) as compared to individual isolates. Pot scale experiment (enriched with 100 mg Cu kg−1) showed inoculation of Helianthus annuus seeds with consortium of TR15a + TR15c had significantly (p < 0.05) improved seed germination by 32%, total dry biomass by 64%, root Cu by 47% and shoot Cu by 75% as compared to uninoculated control whereas 0.2−7 fold higher results were observed for above stated parameters as compared to four individual isolates studied. The result suggests consortium of ACC deaminase producing Pseudomonas sp. TR15a and siderophore producing B. aerophilus TR15c could play a vital role in enhanced Cu uptake and improvement of biomass and may provide a better alternative for decontamination of Cu contaminated natural ecosystem than individual isolates.
Display omitted
•Pseudomonas sp. TR15a and Bacillus aerophilus TR15c produced high ACC deaminase.•Consortium of TR15a + TR15c showed high resistance to multiple metals and antibiotics.•Siderophore production and ACC deaminase activity were improved for consortium.•Growth and Cu concentration in sunflower enhanced after application of consortium.•Consortium act as better phytostabilizer than single inoculant for Cu remediation.
Rhizodeposition affects the microbial community in the rhizosphere, and microbial composition and activity may therefore differ in soil depending on plant species. We hypothesised that these ...differences increase over the plant growth period because roots occupy larger soil volumes and release more rhizodeposits. We tested how such medium-term responses of the microbial community can be explained by the short-term utilisation of root exudates. To test this we analysed super(15)N incorporation into microbial biomass, phospholipid fatty acid (PLFA) composition and super(13)C incorporation into the PLFAs of specific microbial groups in soil under white clover (Trifolium repens L.) and ryegrass (Lolium perenne L.) following leaf-labelling with super(13)C-bicarbonate and super(15)N-urea. In this way microbial N and super(15)N and the composition of PLFAs reflect the medium-term (two months) response of microorganisms to rhizodeposits, whereas the super(13)C-label of the PLFAs reflects the short-term (one week) utilisation of root exudates following labelling of shoots. In the medium term, microbial biomass N and super(15)N were greater under the ryegrass, whereas total PLFA was higher under white clover. The relative abundance of fungi and actinomycetes was unaffected by plant species, but pool of Gram-negative and Gram-positive bacteria was greater under white clover at the 10 percent significance level. In the short term, microorganisms more actively utilised fresh exudates ( super(13)C-labelled) of ryegrass than of white clover. We expected ryegrass exudates initially to be incorporated into bacterial PLFA and into fungi over time, but surprisingly fungi had the highest utilisation of ryegrass-derived C over the week. At 0-5 cm soil depth, white clover exudates were utilised only by bacteria, whereas fungi dominated at 5-15 cm. This reflects differences in the quality of white clover exudates or differences in the microbial community composition at the two depths. We conclude that despite clear short-term differences in microbial response to the exudates of white clover and ryegrass, this is only to a limited extent transferred into medium-term defects on the composition of the microbial communities under the two plant species. Hence, our study showed that different short-term C utilisation patterns may lead to similar medium-term responses of the microbial community.
Summary
Allopolyploid speciation and chemical defense diversification are two of the most characteristic features of plant evolution; although the former has likely shaped the latter, this has rarely ...been documented. Here we document allopolyploidy‐mediated chemical defense evolution in the origin of cyanogenesis (HCN release upon tissue damage) in white clover (Trifolium repens).
We combined linkage mapping of the loci that control cyanogenesis (Ac, controlling production of cyanogenic glucosides; and Li, controlling production of their hydrolyzing enzyme linamarase) with genome sequence comparisons between white clover, a recently evolved allotetraploid, and its diploid progenitors (Trifolium pallescens, Trifolium occidentale).
The Ac locus (a three‐gene cluster comprising the cyanogenic glucoside pathway) is derived from T. occidentale; it maps to linkage group 2O (occidentale subgenome) and is orthologous to a highly similar cluster in the T. occidentale reference genome. By contrast, Li maps to linkage group 4P (pallescens subgenome), indicating an origin in the other progenitor species.
These results indicate that cyanogenesis evolved in white clover as a product of the interspecific hybridization that created the species. This allopolyploidization‐derived chemical defense, together with subsequent selection on intraspecific cyanogenesis variation, appears to have contributed to white clover’s ecological success as a globally distributed weed species.
Abstract
The clover seed weevil, Tychius picirostris Fabricius, a serious pest of white clover, Trifolium repens L., grown for seed in western Oregon, causing feeding damage to flowers and developing ...seeds. Since 2017, white clover seed producers have anecdotally reported T. picirostris control failures using foliar pyrethroid insecticide applications. This mode of action (MoA) is an important chemical control option for T. picirostris management. To evaluate insecticide resistance selection to pyrethroids (bifenthrin) and other MoAs labeled for T. picirostris management (malathion and chlorantraniliprole), adult populations were collected from 8 commercial white clover grown for seed fields in the Willamette Valley, OR, in 2022 and 2023. Among collected Oregon populations, very high resistance ratios (RR50 = 178.00–725.67) were observed to technical grade bifenthrin and low to high resistance ratios (RR50 = 7.80–32.80) to malathion in surface contact assays compared to a susceptible Canadian field population. Moreover, >2.73 times the labeled rate of formulated product containing bifenthrin as the sole MoA was required to kill >50% of T. picirostris in topical assays. Synergistic assays with a mixed-function oxidase inhibitor, an esterase inhibitor, and a glutathione-S-transferase inhibitor revealed phase I and II detoxification enzymes are present in Oregon T. picirostris populations and confer metabolic resistance to bifenthrin. This is the first report of T. picirostris insecticide resistance selection to pyrethroid and organophosphate insecticides. Results will inform continued monitoring and insecticide resistance management strategies to slow the evolution of T. picirostris insecticide resistance selection in Oregon’s white clover seed production.
The presence of plants induces strong accelerations in soil organic matter (SOM) mineralization by stimulating soil microbial activity – a phenomenon known as the rhizosphere priming effect (RPE). ...The RPE could be induced by several mechanisms including root exudates, arbuscular mycorrhizal fungi (AMF) and root litter. However the contribution of each of these to rhizosphere priming is unknown due to the complexity involved in studying rhizospheric processes. In order to determine the role of each of these mechanisms, we incubated soils enclosed in nylon meshes that were permeable to exudates, or exudates & AMF or exudates, AMF and roots under three grassland plant species grown on sand. Plants were continuously labeled with 13C depleted CO2 that allowed distinguishing plant-derived CO2 from soil-derived CO2. We show that root exudation was the main way by which plants induced RPE (58–96% of total RPE) followed by root litter. AMF did not contribute to rhizosphere priming under the two species that were significantly colonized by them i.e. Poa trivialis and Trifolium repens. Root exudates and root litter differed with respect to their mechanism of inducing RPE. Exudates induced RPE without increasing microbial biomass whereas root litter increased microbial biomass and raised the RPE mediating saprophytic fungi. The RPE efficiency (RPE/unit plant-C assimilated into microbes) was 3–7 times higher for exudates than for root litter. This efficiency of exudates is explained by a microbial allocation of fresh carbon to mineralization activity rather than to growth. These results suggest that root exudation is the main way by which plants stimulated mineralization of soil organic matter. Moreover, the plants through their exudates not only provide energy to soil microorganisms but also seem to control the way the energy is used in order to maximize soil organic matter mineralization and drive their own nutrient supply.
•Role of root exudates, AMF and roots in rhizosphere priming effect (RPE) was disentangled.•Differentially permeable meshes and continuous 13C labeling of grassland monocultures were used.•Root exudation induced 58–96 % of total RPE whereas the rest was contributed by roots.•RPE efficiency was 3–7 times higher for exudates than roots.•Microbial allocation of plant C to mineralizing activity than growth explains higher RPE for exudates.
Aims: The objectives of this study were to compare techniques for measuring biological nitrogen fixation (BNF) and to assess how fertiliser N input affects the balance between BNF and sustainable ...herbage production on perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.) grassland. Methods: Biological N fixation and herbage production by white clover based grassland was measured in 2011 and 2012 under four nominal annual fertiliser N inputs: 0 (0N), 86 (86N), 140 (140N) and 280 kg ha super(-1) (280N). Biological N fixation was measured using the super(15)N isotope dilution and super(15)N natural abundance techniques under all fertiliser N inputs and also using the nitrogen difference technique under 0N. Results: The two super(15)N techniques produced similar annual estimates of above-ground BNF across the range of fertiliser N inputs. Fertiliser N input resulted in higher herbage dry matter yield, but reduced annual BNF which averaged 80, 64, 66 and 47 kg ha super(-1) on 0N, 86N, 140N and 280N, respectively, across both techniques and years. Conclusions: The two super(15)N techniques were comparable in estimating BNF. Intermediate fertiliser N inputs achieved a balance between minimising detrimental impact on BNF and potential N loss while producing more herbage DM yield than clover swards receiving no fertiliser N.
Trifolium repens L. (T. repens) is considered a potential phytoremediation species due to its large biomass and ability to accumulate and tolerate heavy metals. Lead (Pb) is an important heavy metal ...pollutant that can affect plant growth, photosynthesis, and enzyme activity. However, response mechanism of microorganisms in three root niches of metal tolerant plants to Pb is not completely understood. Therefore, in this study, a Pb poisoning model of T. repens was established with a Pb gradient (0, 1000 mg/kg, 2000 mg/kg, and 3000 mg/kg), and was used to evaluate growth and physiological responses, as well as enrichment and transport coefficients in T. repens, and explore the characteristics of rhizosphere soil and microbial composition of three root niches. We found that Pb stress caused oxidative injury, and inhibited photosynthesis in T. repens. 16S rDNA sequencing analysis showed that the richness of microbial communities in bulk soil was higher than that in rhizosphere soil both under Pb stress and Pb nonstress conditions. Moreover, Proteobacteria was dominant phylum in bulk and rhizosphere soils, and Proteobacteria and Cyanobacteria were dominant phylum in endophytic bacteria. For the first time, we systematically investigated the response of Pb from bulk soil to plant leaves. The results showed that microbial interaction existed between bulk and rhizosphere soil. Rhizosphere bacterium Haliangium was positively correlated with urease activity and soil nutrients. Endophytic bacterium Pseudomonas was positively correlated with plant biomass and played an important role in Pb tolerance of T. repens. In addition, endophytic bacteria formed complex correlation networks with growth and physiological indexes of both root and shoot, moreover the network in root was more complicated. Taken together, Pb stress dose-dependently inhibited the growth of plants. This study provided a theoretical basis for the further development of microbial cooperation with plant remediation of heavy metal contaminated soil.
Display omitted
•The richness of microbial communities in bulk soil was higher than that in rhizosphere soil both under Pb stress and Pb non stress conditions.•Proteobacteria and Cyanobacteria were dominant phylum in endophytic bacteria at all Pb levels.•Rhizosphere bacteria Haliangium was positively correlated with urease and soil nutrients under Pb pollution.•Endophytic bacterium Pseudomonas was positively correlated with plant biomass.•Correlation network formed between endophytic bacteria and growth physiological indexes in root was complex than that in shoot.
Backgrounds and aims: N rhizodeposition by legumes leads to enrichment of N in soils and in companion plants. N rhizodeposition can be divided into two major components, root exudation and root ...senescence. Our aim was to quantify N root exudation in white clover (Trifolium repens L.) through an estimation of short-term N rhizodeposition and to assess its impact on N transfer to companion perennial ryegrass (Lolium perenne L.) grown in mixture with clover. Method: super(15)N sub(2) provided in the root atmosphere for 3 days was used to estimate transfer of symbiotically fixed nitrogen (SFN) to the growing medium by clover grown in pure stand and to ryegrass by clover grown in mixture for 2 months. Results: The proportion of N rhizodeposited over the 3 days increased from 3.5 % of SFN in pure stand to 5.3 % in mixture. The super(15)N-enrichment of ammonium from the adhering substrate shows that a part of the rhizodeposited N was released in the form of ammonium. 4 % of the rhizodeposited N was taken up by ryegrass during the labelling period. Conclusions: This study showed a significant contribution of root N exudation to the total N rhizodeposition of legumes and in the transfer of N to grasses.
Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from ...6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale.
PDF
